Conventional I/Q modulators are used in transmitting means for carrier-frequency transmission systems, e.g. transmitters for digital broadcasting, and in base stations for mobile communications.
One example of such a transmitting means is shown in FIG. 4. The transmitting means 400 comprises an I/Q modulator 402 with predistortion, the I/Q modulator 402 comprising a first input connected to the input of the transmitting means 400, and an output. The first input of the I/Q modulator has an I/Q signal applied thereto. The output of the I/Q modulator is connected to a first input of a first mixer 404. A second input 404 of the mixer is connected to an oscillator 406. An output of the mixer 404 is connected to an input of an amplifier 408. An output of the amplifier 408 is connected to an antenna 410. The amplifier 408 and the antenna 410 have arranged between them a decoupling means 412 which is connected to an attenuator 414. An output of the attenuator 414 is connected to a first input of a second mixer 416. A second input of the second mixer 416 is also connected to the oscillator 406. An output of the mixer 416 is connected to an input of an I/Q demodulator 418. An output of the I/Q demodulator 418 is connected to a first input of a comparator 420. A second input of the comparator 420 is connected to an output of a delay element 422. An output of the comparator 420 is connected to a second input of the I/Q modulator 402. An input of the delay element 422 is connected to the first input of the I/Q modulator 402 and to the input of the transmitting means 400, respectively.
The decoupling means 412, the attenuator 414, the second mixer 416, the I/Q demodulator 418 and the comparator 420 define a feedback for determining the distortion parameters.
In the following, the mode of operation of the transmitting means according to FIG. 4 will be described briefly. An I/Q signal, which is e.g. a message-carrying baseband signal, is modulated onto a carrier signal by means of the I/Q modulator. In order to compensate e.g. distortions of the first mixer 404 and of the amplifier 408, the I/Q modulator additionally performs a predistortion of the I/Q signal. This is important especially when transmit signals with a non-constant envelope are used. Such a non-constant envelope occurs e.g. in cases in which amplitude-modulated instead of frequency-modulated signals are used so as to achieve a higher spectral efficiency of the modulation method. The non-constant envelope of the transmit signal causes in connection with the non-linearities of the first mixer 404 and of the amplifier 408 disturbances outside the useful frequency band. These disturbances are referred to as adjacent-channel emissions and should typically not exceed an application-specific limit value.
The predistorted output signal of the I/Q modulator 402 is fed to the first mixer 404 in which the signal is up-converted with the aid of the oscillator 406. The up-converted signal is then amplified by the amplifier, e.g. a travelling wave tube, and sent to the antenna 410 and transmitted.
Part of the signal sent to the antenna 410 is previously tapped off by the decoupling means 412 and, for further processing, it is attenuated by the attenuator 414 so as to reverse the amplification of the amplifier. The tapped-off attenuated signal is fed to the second mixer 416 for down-conversion, and the down-converted signal is then fed to the I/Q demodulator so as to be demodulated into an I/Q signal. The demodulated I/Q signal now carries the information on the distortion caused in the original I/Q signal e.g. by the first mixer 404 and the amplifier 408. When this I/Q signal is supplied to the comparator 420, the comparison between the original I/Q signal and the demodulated, distorted I/Q signal will provide an information indicating what predistortion of the I/Q modulator 402 has to be chosen so that the distortions caused by the first mixer 404 and the amplifier 408 can be compensated for in the best possible way.
A feature which is important to the comparison is that the original I/Q signal is delayed in time by the delay element 422 prior to the comparison in the comparator 420 so that the original I/Q signal is actually the signal which caused the predistorted I/Q signal. This method of adjusting the predistortion of the I/Q modulator 402 in dependence upon a comparison is referred to as adaptive predistortion.
An example of such an adaptive predistortion is described in U.S. Pat. No. 5,049,832. U.S. Pat. No. 5,049,832 discloses an amplifier linearization of an amplifier circuit by adaptive predistortion in the case of which an input signal for a power amplifier of the amplifier circuit is derived from an input modulation signal of the amplifier circuit by predistortion, i.e. the input signal of the power amplifier is predistorted so as to achieve a linear amplification of the input signal by the power amplifier.
The predistortion of the input modulation signal is adjusted via a table, which is addressed in dependence upon the square of the amplitude of the input modulation signal, the contents of the table being continuously updated so that variations of the distortion caused by the power amplifier can be taken into account during the predistortion caused by the I/Q modulator.
In the following, a conventional I/Q modulator with predistortion will be described. Making use of signals which are represented as a complex function, FIG. 5 shows the principle of an I/Q modulator 500 with predistortion of the I/Q signal. The I/Q modulator 500 has a first multiplier 502 which is connected to a second multiplier 504. The second multiplier 504 is connected to an element 506 for forming the real component.
A first input of the first multiplier 502 has applied thereto a complex I/Q signal or a baseband signal comprising an I component and a Q component:x(t)=i(t)+jq(t)  equa. 1
A second input of the first multiplier 502 has applied thereto a complex predistortion signal.p(t)=p1(t)+jp2(t)  equa. 2
The multiplier 502 multiplies the I/Q signal by the predistortion signal and supplies at one output a predistorted I/Q signal.xp(t)=x(t)·p(t)  equa. 3
The real component and the imaginary component of the predistorted I/Q signal have the following form:ip(t)=Re{xp(t)}=i(t)·p1(t)·q(t)·p2(t)  equa. 4qp(t)=Im{xp(t)}=i(t)·p2(t)+q(t)·p1(t)  equa. 5
The predistorted I/Q signal is fed to a first input of the second multiplier 504 and multiplied by a carrier signal applied to a second input, whereby it is applied to a carrier having an angular frequency ω0 so as to produce a complex output signal at an output of the second multiplier 504. The complex output signal is fed to the element 506 for forming the real component so as to supply a real output signal at one output of the element 506 for forming the real component.y(t)=Re{xp(t)·ejω0t}=ip(t)·cos ω0t−qp(t)·sin ω0t   equa. 6
As can be seen from equations 4 and 5, four multiplications, i.e. four multipliers in a circuit, are required for calculating the predistorted I/Q signal. In addition, two multiplications, i.e. two multipliers, are required for multiplying the carrier signal by the predistorted I/Q signal, as can be seen from equation 6. It follows that six multipliers are required for realizing in circuitry the I/Q modulator with predistortion of the I/Q signal according to FIG. 5.
FIG. 6 shows a conventional I/Q modulator 600 with predistortion of the I/Q signal or predistortion of the baseband signal. The I/Q modulator comprises a first input 602 and a second input 604 and an output 606. The first input 602 is connected to a first input of a first multiplier 608 and a first input of a second multiplier 610. The second input 604 of the I/Q modulator 600 is connected to a first input of a third multiplier 612 and a first input of a fourth multiplier 614. A second input of the first multiplier 608 and a second input of the third multiplier 612 are connected to a first output of a means 616 for producing a predistortion signal. A second input of the second multiplier 610 and a second input of the fourth multiplier 614 are connected to a second output 620 of the means 616 for producing a predistortion signal. An output of the first multiplier 608 is connected to a first input of a first adder 622. An output of the fourth multiplier 614 is connected to a second input of the first adder 622. An output of the second multiplier 610 is connected to a first input of a second adder 624 and an output of the third multiplier 612 is connected to a second input of the second adder 624.
An output of the first adder 622 is connected to a first input of a fifth multiplier 626, and an output of the second adder 624 is connected to a first input of a sixth multiplier 628. A second input of the fifth multiplier 626 is connected to a first output 632 of a means 630 for producing a carrier signal. A second input of the sixth multiplier 628 is connected to a second output 634 of the means 630 for producing a carrier signal.
An output of the fifth multiplier 626 is connected to a first input of a third adder 636. An output of the sixth multiplier 628 is connected to an inverting second input of the third adder 636. An output of the third adder 636 is connected to the output 606 of the I/Q modulator.
In the following, the mode of operation of the I/Q modulator 600 with predistortion of the I/Q signal according to FIG. 6 will be described briefly. The first input 602 of the I/Q modulator 600 has applied thereto the I component or real component of the I/Q signal, and the second input 604 of the I/Q modulator 600 has applied thereto the Q component or imaginary component of the I/Q signal.
The first output 618 of the means 616 for producing the predistortion signal has applied thereto the real component p1(t) of a predistortion signal p(t). The second output 620 of the means 616 for producing a predistortion signal p(t) has applied thereto the imaginary component p2(t) of the predistortion signal.
The first multiplier 608 performs the first multiplication according to equation 4. The fourth multiplier 614 performs the second multiplication according to equation 4. Furthermore, the first adder 622 forms the sum of equation 4 so as to obtain the real component ip(t) of the predistorted I/Q signal xp(t). The second multiplier 610 performs the first multiplication according to equation 5. The third multiplier 612 performs the second multiplication according to equation 5. In addition, the second adder 624 forms the sum according to equation 5 so as to produce the imaginary component qp(t) of the predistorted I/Q signal xp(t).
The first output 632 of the means 630 for producing a carrier signal has applied thereto a first subcomponent, here the real component of the carrier signal, which is e.g. a cos function. The second output 634 of the means 630 for producing the carrier signal has applied thereto a second subcomponent, here the imaginary component of the carrier signal, which is a subcomponent, e.g. a sin function, that is substantially orthogonal to the first subcomponent.
In addition, the fifth multiplier 626 performs the first multiplication according to equation 6, whereas the sixth multiplier 628 performs the second multiplication according to equation 6. Finally, the third adder 636 forms the difference according to equation 6 so as to produce the real output signal y(t) at the output 606 of the I/Q modulator.
In the case of modern transmitting means the predistortion and the I/Q modulation are carried out digitally. In view of the large bandwidth and the high precision demands of modern transmission methods, such as e.g. W-CDMA (W-CDMA=Wideband Code-Division Multiple Access), fast digital multipliers having a high resolution, typically 14 bits, are required for this purpose.
FIG. 7 shows a time-discrete i.e. digital realization of the conventional I/Q modulator with predistortion of the I/Q signal according to FIG. 6. The I/Q signal, the carrier signal, the predistortion signal and the output signal are represented by sampled values at intervals TA=1/fA. fA is the sampling rate, wherein t=n·TA and Ω0=ω0·TA. n is the sampling parameter.
From equations 4, 5 and 6, it follows that:ip(n)=i(n)·p1(n)−q(n)·p2(n)  equa. 7qp(n)=i(n)·p2(n)+q(n)·p1(n)  equa. 8y(n)=ip(n)·cos Ω0n−qp(n)·sin Ω0n  equa. 9
The means 730 for producing a carrier signal is now e.g. a numeric oscillator (NCO) of the type shown in FIG. 8. The numeric oscillator comprises a phase accumulator, which forms the phase Ω0n, and a sine table which is addressed by the phase. The two orthogonal subcomponents of the carrier signal are supplied at the first output 732 and at the second output 734 of the means 730 for producing a carrier signal. The first subcomponent of the carrier signal is here the sin function from which the second subcomponent, the cos function, can easily be calculated in the numeric oscillator by shifting the phase by a quarter period π/2.
One disadvantage of the conventional digital I/Q modulator with predistortion of the I/Q signal is that six fast digital multipliers with high resolution are required for processing the I/Q signal.
As far as a realization is concerned, this means that a large number of gates will be necessary and that power consumption will be high.
DE 198 32 116 A 1 discloses a method of linearly predistorting a digitized signal, wherein the signal is to be transmitted with the aid of a multi-carrier method and wherein the carrier frequencies used in this multi-carrier method and forming the carrier spectrum are acted upon by correction values.
DE 196 21 388 C 2 discloses a method of predistorting a signal which is to be transmitted over a non-linear transmission path, and a circuit arrangement for executing this method. The envelope of a signal is detected, whereupon quantized envelope values are formed. Following this, complex predistortion coefficients are formed, which depend on the quantized envelope values and on a previously detected transfer function of the non-linear transfer path, whereupon the signals to be transmitted via the non-linear transmission path are subjected to complex weighting with the complex predistortion coefficients so that the distortion caused by the non-linear transmission path will be compensated for to a large extent with respect to magnitude and phase.
DE 36 43 689 A 1 discloses a method and an arrangement for digitally predistorting 16-QAM signals. For compensating transmission-dependent distortions of the digital quaternary baseband signals of a transmission executed by means of the 16-QAM method, the symbol pulse duration is subdivided into a number of n support points. An optimizing method provides the amplitude values of the support points which are adapted to the actual channel characteristics. The amplitude values are stored as coefficients in read-only memories and used for predistorting a transmit signal in an QAM modulator.